1. Field of the Invention
The present invention relates to a process of removing trace levels of acidic impurities from a photoacid generator solution. In particular, the present invention is directed to a process of removing trace levels of acidic impurities from photoacid generator solutions by contacting the photoacid generating solution with an amine-containing ion exchange resin, as well as purified photoacid generator solutions made by the process.
2. Description of the Art
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components such as in the fabrication of integrated circuits and printed wiring board circuitry. Generally in these processes, a thin coating or film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits or aluminum or copper plates of printed wiring boards. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure of radiation. This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam, and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed area of the coated surface of the substrate.
Two types of photoresist compositions are generally used negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g., a crosslinking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to a developing solution. Thus, treatment of an exposed negative-working resist with a developer solution causes removal of the nonexposed areas of the resist coating and the creation of a negative image in the photoresist coating, and thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited. On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the resist composition exposed to the radiation become more soluble to the developer solution while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working resist with the developer solution causes removal of the exposed areas of the resist coating and the creation of a positive image in the photoresist coating. Again, the desired portion of the underlying substrate surface is uncovered.
After this development operation, the now partially unprotected substrate may be treated with a substrate-etchant solution or plasma gases and the like. This etchant solution or plasma gases etch the portion of the substrate where the photoresist coating was removed during development. The areas of the substrate where the photoresist coating still remains are protected and thus, an etched pattern is created in the substrate material which corresponds to the photomask used for the image-wise exposure of the radiation. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a clean etched substrate surface. In some instances, it is desirable to heat treat the remaining resist layer after the development step and before the etching step to increase its adhesion to the underlying substrate and its resistance to etching solutions.
Positive-working photoresist compositions are currently favored over negative-working resists because the former generally have better resolution capabilities and pattern transfer characteristics.
Chemically amplified photoresists are commonly employed today in advanced lithographic processes and generally contain a photoacid generator (also known as PAGs), a polymer and, optionally, a dissolution inhibitor. Either the polymer or the dissolution inhibitor or both have acid-labile groups attached thereof. In such systems, the photogenerated acid causes removal of acid-labile protecting groups on the polymer and/or dissolution inhibitor. The solubility of the deprotected areas in aqueous alkaline solutions is much greater than the areas having the protect ed polymer or dissolution inhibitor. This change in solubility upon exposure to incident radiation is the basis for positive working chemically amplified photoresists.
While the photogeneration of acid is necessary for chemically amplified photoresist systems, the presence of trace amounts of acidic impurities in the initial photoresist may result in premature deprotection of the polymer and/or dissolution inhibitor. As a result, the presence of acidic impurities in photoacid generators adversely affects the shelf-life of the photoresist and the overall lithographic performance.
Traditionally, trace levels of acidic impurities have been removed from PAGs by water washing using two methods. One method generally involves exhaustively rinsing a solid PAG with deionized water. Another method involves forming a dilute solution of PAG in a water-immiscible solvent such as ethyl acetate, methylene chloride, chloroform and the like, and washing the PAG solution with water using extraction techniques. However, these methods are time-consuming and generate large volumes of aqueous and/or solvent waste which can be difficult to dispose.
The prior art also describes numerous techniques for reducing the amount of impurities in chemically amplified photoresist components. Some of these teachings include the following:
U.S. Pat. No. 5,472,616 issued Dec. 5, 1995 to Szmanda et al. and assigned to Shipley Company, claims a process for removing anionic contaminants from an organic solution containing a base-labile solute, comprising the steps of contacting the organic solution with an anion exchange resin that has had strong basic groups displaced by weaker anions having a basicity less than that of the hydroxyl anion for a time sufficient to remove essentially all anionic contaminants from solution.
U.S. Pat. No. 5,500,127 issued to Carey et al. on Mar. 19, 1996 and assigned to Rohm and Haas Co. discloses a method for removal of cationic and anionic contaminants from an organic solution of an acid catalyzed photoresist composition containing one or more acid labile components subject to reaction in the presence of a strong acid, the process comprising the steps of providing an organic solution containing one or more acid labile components, providing a weak acid cation exchange resin having a pKa such that the weak acid exchange groups on the resin do not react with acid labile components of the photoresist, providing a strong base anion exchange resin, and contacting the organic solution with the cation exchange resin and the anion exchange resin for a time sufficient to remove essentially all dissolved contaminants from solution without causing reaction between the acid labile components of the photoresist composition and the cation exchange resin.
U.S. Pat. No. 5,518,628 issued on May 21, 1996 to Carey and assigned to Shipley Company claims a process for removing ionic contaminants from an organic solution containing acid and base labile solutes, the process comprising the steps of providing a mixed bed of cation and anion exchange resins which has been treated by contact with an ammonium salt of a weak acid and contacting the organic solution with the treated bed of exchange resins for a sufficient time to remove essentially all ionic contaminants from the solution.
U.S. Pat. No. 5,525,315 issued to Burke on Jun. 11, 1996, and assigned to the Shipley Company, claims a process for removing heavy metal ions contained in an organic solution of one or more photoresist components, comprising the steps of washing a chelating cation exchange resin with an acid to remove essentially all sodium ions therefrom and rinsing the acid washed chelating cation exchange resin with water where the water effluent has a pH varying between 1 and 6, and contacting the organic solution containing heavy metal ions with the acid washed chelating cation exchange resin or a time sufficient to reduce the concentration of the heavy metal ions contained in the organic solution.
Reduction of acidic impurities from the photoacid generating component of today's advanced photoresist compositions is critical to their performance and shelf-life. Specifically, there is now a need to produce solutions of photoacid generators that contain less than 15 parts per million (ppm) of acidic impurities. The process of the present invention is an answer to that need.